Solvent extraction of mercaptans



Oct. 26, 1948. H. E. BRENNAN SOLVENT EXTRAC'I'ION 0F MERCAPTANS 2 Sheets-Sheet l Filed Oct. 22. 1945 ATTORNE Oct. 26, 1948. H. E. BRENNAN SOLVENT EXTHACTION OF MEHCAPTANS 2 Sheets-Sheet Filed Oct. 22, 1945 Mmmwmwwww www o "HO BDUVHD NI Hd'lnS NVLdVJUSW Z LHSIBM Patented er. 26, 1948 SOLVENT EXTBACTION OF MEBCAPTANS Harry E. Drennan, Hot Springs, Ark., assignor to Phillips Petroleum Company, a corporation of ol Delaware Application October 22, 1945. Serial No. 623,798

9 Claims. l

This invention relates to the preparation of mercaptans. In one of its more speciilc aspects the invention relates to the segregation oi substantially pure alkyl mercaptans from mixtures oi' the same with hydrocarbons. A preferred embodiment relates to the application of solvent extraction to crude mercaptan oils containing at least four Weight per cent mercaptan'suliur to eil'ect the separation of substantially pure mercaptans from closely related hydrocarbon types` A specic modification of the present invention relates to the treatment of mercaptan-h'ydrocarbon mixtures with aqueous alcoholic alkaline solvents, with subsequent extractive treatment of the mercaptan-rich alkaline solvent to eilectively purify and recover said mercaptans.

In the past, interest in the separation of mercaptans from hydrocarbon oils has been almost exclusively confined to petroleum distillates in which small concentrations of mercaptans constituted a nuisance in the nished products. It has long been customary to improve the odor of hydrocarbon distillates by the various sweetening processes in which mercaptans are chemically altered, but not removed. More recently, combustion characteristics of gasolines have been reported as beneting from complete removal of mercaptans from the distillates. One such process for the removal of mercaptans from hydrocarbons involves the use of causticmethanol extraction. However, in the treatment of gasoline. the mercaptan sulfur concentration seldom exceeds a few tenths oi.' one per cent of the hydrocarbon material, and subsequent recovery of hydrocarbon-free mercaptans is ordinarily not attempted.

Recent developments in the synthetic rubber industry have created a demand for pure mercaptans of relatively high molecular weight, thus presenting problems oi' segregation and recovery of such mercaptans not heretofore encountered. Mercaptans suitable for use as modifiers in the manufacture oi buna-type synthetic rubbers are ordinarily selected from tertiary types having from about 12 to 14 carbon atoms per molecule. Other heavy mercaptans, such as those of eight and more carbon atoms, are becoming of great interest commercially. Such mercaptans may advantageously be prepared by the catalytic addition of hydrogen sulde to the corresponding olens. but inisuch cases the crude product is almost invariably a mixture of the desired mercaptans with appreciable quantities of unreacted olefin. The actual mercaptan content of typical synthetic crude mercaptan oils may range from (Cl. 26o-809) about 20 to about 60 weight per cent, with a mercaptan sulfur content, for example in the case of Cn-Cio mercaptans, oi i'rom about 4 to about 12 weight per cent. These various crude mercaptan oils must be further concentrated to mercaptan assays of at least to 95 per cent before employment as modifying reagents in emulsion polymerizations leading to the production of buna-type synthetic rubbers. or for many other purposes.

The application oi' vacuum fractional distillation to such crude mercaptan oils may result in the recovery of a relatively pure mercaptan product: however, the diiiiculties inherent in high-vacuum operations are well known. The existent art pertaining to the solvent extraction oi' very low concentrations of mercaptans from petroleum distillates in inadequate to solve the highly speciiic problem o! segregation and recovery of relatively pure mercaptans. especially those in the higher molecular weight range, from mercaptan concentrates. This is emphasized by the number oi publications on this subject describing solvent regeneration by oxidizing or otherwise destroying the mercaptans and by steam distillation. The present invention is particularly applicable to concentrates of relatively non-volatile and somewhat thermally unstable mercaptans which must be recovered in a pure state, and is preferably utilized for the recovery of mercaptans having four or more carbon atoms per molecule.

An important object of this invention is to prepare pure mercaptans.

Another object is to separate alkyl mercaptans in substantially pure form from admixture with hydrocarbons or other substantially neutral or inert organic liquids.

A further object is to subject mercaptan concentrates formed by the catalytic addition oi' hydrogen sulfide to oleilns. and which contain unreacted olefins, to treatment leading to the ultimate recovery of pure mercaptans free from residual hydrocarbons.

Yet another object is to subject an equeous alcoholic alkali metal hydroxide solution o! mercaptans to a novel series oi treatments resulting in the production of pure mercaptans.

A further object of the present invention is to provide specific solvent compositions suitable for optimum results in the extractive treatment oi crude mercaptan oils having variable mercaptan concentrations.

A still further object is to provide i'or the recovery of substantially pure mercaptans from crlde mercaptan oils by a novel combination of extraction operations by means of which the desired mercaptans are transferred from an original high-boiling carrier to a selected lowboillng carrier from which the pure mercaptans can be conveniently recovered by simple evaporation of the final carrier solvent.

These and other objects and advantages of the invention will be apparent, to one skilled in the art, from the accompanying disclosure and cliscussion.

The process of my invention ln preferred embodiment employs as raw feed a crude mercaptan oil of relatively high mercaptan content as compared with sour petroleum distillates. The mercaptan composition of such oils may vary from about 20 per cent to about 60 per cent or even more. A primary extraction procedure is carried out with an appropriate aqueous alcoholic alkali metal hydroxide solution, preferably in the absence of air. Choice of solvent and conditions will be discussed later in detail. Methanol, ethanol, propanol. or other alcohol may be used, though methanol is much preferred. Likewise, sodium hydroxide is preferred among the various alkali metal hydroxides which may be utilized, which include the hydroxides of potassium, lithium, etc. The primary extraction may be carried out as a multistage process with countercurrent flow of oil and solvent, or the extraction may be effected in a packed column with countercurrent flow. In either event, the final rich solvent ellluent will be substantially saturated with a mercaptan-oil material comprising a major proportion of mercaptan and a minor proportion of neutral oil, while the lean oil phase will have a mercaptan content of say from to 2 per cent, depending on the initial mercaptan content and on the number of stages employed in the primary extraction. It is an important feature of the invention that at least 90% of the mercaptan content of the crude mercaptan oil is extracted by the primary solvent. Actually, the mercaptan present in the solvent phase is believed to be largely ln the form of soluble alkali metal mercaptides, although the exact proportions of mercaptides and of free mercaptans in the rich alkaline solvent are not known.

The next, or secondary extraction, step of the process of my invention comprises the treatment of the mercaptan-rich solvent phase in a certain manner to recover pure mercaptans therefrom. This is effected through the action of a liquid hydrocarbon extracting medium, which preferably is contacted with the rich solvent in at least two stages, separate portions of the hydrocarbons being used in different Stages. Preferably used is a low-boiling sulfur-free liquid hydrocarbon, such as propane, butane, pentane, or the like, which may be termed a secondary solvent. The first stage is carried out with extraction conditions particularly chosen so that selective removal of residual heavy oil contaminant from the alkaline solvent is accomplished, the heavv oil being transferred substantially completely to the lowboiling hydrocarbon solvent, generally along with a small portion of the mercaptan. This treatment purifles the alkaline solvent, and may be effected in one or more stages as desired, although a single step is ordinarily quite adequate. After this complete removal of residual heavy oil, the alkaline solvent is contacted with another portion or portions of the low-boiling secondary solvent in one or more stages under different conditions regulated to transfer the mercaptan from 4 the former to the latter. The thus-denuded alkali solvent is recycled to the primary extraction step.

The final operational step of my process is the recovery of product and of hydrocarbon solvent. The hydrocarbon solvent is reclaimed by simple evaporation or distillation. The stream from the rst stage secondary extraction yields a residue comprising heavy oil and some mercaptan, which residue is preferably recycled to the primary extraction step for ultimate recovery of the mercaptan, while the rich solvent stream from the last stages yields a residue of high purity mercaptan product. Streams from intermediate stages, if any, are recycled to the primary extraction step or yield mercaptan product, de pending on their content of heavy oil. The recovered low-boiling secondary hydrocarbon solvent is combined for reuse in the various steps of the secondary extraction step.

In order to illustrate and exemplify my invention, the accompanying drawings are provided. Figure 1 is a simplified flow diagram in somewhat schematic form, showing one arrangement of apparatus elements and flow of materials therethrough suitable for practicing the invention in a specific and preferred embodiment. Figure 2 is a series of curves inter-relating the solvent composition with the mercaptan sulfur content of the crude product being treated and the temperature of extraction. With respect to Figure 1, it will be appreciated that the drawing is only schematic and not necessarily drawn to scale, and that all of the elements of auxiliary apparatus such as valves, liquid level controllers, temperature controllers, heaters, coolers, etc. are not shown in order to avoid burdening the drawing with too much detail, for these and other elements are readily understood and supplied by one skilled ln the art.

Referring now to Figure 1, crude mercaptan oil is charged through line I and rate control valve 2 to the inlet side of mixing pump 3. Simultaneously a regulated flow of aqueous caustic-methanol solvent from settling tank 8 is introduced into line I via line 2l and the rate controlling valve 20. The solvent and oil phases, after thorough commingling in pump 3, are discharged through pipe 4 into settling tank 5 where gravity separation of the phases occurs. The mercaptanbearing oil phase is then Withdrawn through line E and charged to pump 1 for the second stage extraction, While the solvent phase is continuously removed to the rich solvent tank 2l by way of valve 22 and transfer line 23. Solvent for the second stage extraction is derived from settling tank I3 and is charged to pump 1 through valve I9 and line I8. The emulsion from pump 1 is discharged through pipe B to settling tank 9 from which the oil phase is removed by line I0 to mixing pump II While the solvent layer is sent to the first stage pump 3 as previously described. The third stage extraction of the mercaptan oil is carried out in mixing pump Il where the now relatively lean oil is contacted With fresh or regenerated caustic-methanol solvent transferred from solvent tank 25 through now-control valve 29, line 26, pump 21 and pipe 28. The thirdstage emulsion is discharged by way of line I2 into tank I3 from which the separated and substantially mercaptan-free oil is passed through pipe I4 to tank I5 for further separation of solvent and oil phases. The mercaptan-denuded oil is finally sent to storage through line I 6 While the once-used solvent in tanks I3 and I5 is sent to the second stage mixing pump 1 through valves Il and l1 and transfer line I8.

The secondary extraction step involving the removal oi' absorbed oil and mercaptan from the aqueous caustic-methanol solvent is illustrated as a two stage operation employing a light hydrocarbon such as pentane, butane or the like as the secondary solvent phase. The mercaptan-containing caustic-methanol aqueous solution from the primary extraction step is withdrawn from tank 24 through ilow-control valve 30 and thence by line 3l to pump 32 where it is thoroughly agitated at a low temperature with a predetermined volume of pentane fiowing from tank 33 via rate-controlling valve 31, line 34, pump 35 and line 38. The resulting emulsion is discharged from the mixing pump through pipe 33 into settling tank 39 where phase separation occurs. The pentane layer, containing mainly absorbed heavy neutral oil along with a minor amount of mercaptan, is continuously removed to storage tank 4I through pipe 48. The caustic-methanol phase which is now substantially free of neutral oil is passed via valve 42 and line 43 to pump 44 where it is extracted at a considerably higher temperature with pentane owing from tank 33 through valve 31, line 34, pump 35, line 33 and line 45. After a suitable contact period, the emulsion is discharged from the pump into line 46 and thence into settling tank 41 for phase separation. The now mercaptan-rich pentane phase is continuously withdrawn to storage tank 33 through pipe 48. Subsequent process steps involving the denuded caustic-methanol phase depend on the quantity of pentane in this phase, which in turn is dependent on the operating conditions. If the proportion of pentane is small, the caustic-methanol phase is pumped to storage tank 25 by way of valve 49, line 50, valve Illl, line 63, pump 64 and line 6B. On the other hand, if the caustic-methanol solution contains a considerable quantity of pentane, valve H is closed and valve III opened, and the solution is transferred via valve 49, pipe 50, valve il, and pipe 2, to pump 5| and thence through heat exchanger 52 to stripping column 53, which is operated in a conventional manner, with additional heat being supplied by means of steam coil 54. 'Ihe pentane is taken overhead through line 56, condenser 51, and line 53 to accumulator tank 59 and thence to pentane storage tank 33 through valve 60 and lines 6I and 82. The now pentane-free aqueous caustic-methanol solvent is removed from the stripping column through valve B5, line 63, pump i4, line 66, and heat exchanger 61 to storage tank 25. The water-caustic-methanol solution in tank 25 is ready for use in the primary extraction operation as described above.

The nal sequence of operations involves the recovery of the mercaptan product and the pentane solvent from the oil-mercaptan-pentane solution accumulated in tank 4| and the main mercaptan-pentane solution in tank 68. The oil-rich pentane solvent is Withdrawn from tank 4| through valve 69 and line 1li by means of pump 1I, with subsequent discharge from said pump into line 12, heat exchanger 13, and stripping column 14, which is operated in a manner known to the art. Heat may be supplied to the column by steam coil 15, and operating temperatures and pressures are selected in accordance with the type of hydrocarbon solvent involved. The hydrocarbon, in this instance pentane, is removed overhead through line 16 and is condensed in condenser 11, from which the condensate is discharged via line 18 into accumulator 18. Pump 8l is used to transfer the recovered pentane from tank 18 through valve 39 and pipes 32 and 92 to storage tank 33. The recovered pentane solvent is then ready i'or recycling to the secondary extraction steps as previously described. 'I'he pentane-free neutral oil-mercaptan blend is removed from column 14 through valve 83, line B4. heat exchanger 85, pump 88, and line 91, and may be partly or completely passed to storage tank 39 through valve 98. A portion or all of this oil is preferably recycled to the primary extraction step by way of line 81 and valve 90 for ultimate complete recovery of the mercaptan content and rejection of the oil content. This may be done continuously, or the oil may be Withdrawn intermittently from storage 89 by way of valve |99, pump 86, line 81, and valve 90.

The mercaptan-rich pentane solvent from the second stage of the secondary extraction step in tank 68 is treated for recovery of the substantially pure mercaptans. The pentane solution is charged to conventional stripping column 98 by way of valve 9|, line 92, pump 93, line 94, and heat exchanger 95. Additional reboiler heat may be furnished to the column by means of steam coil 91 and the column is so operated as to furnish a pentane overhead product and substantially pure mercaptan as the kettle product. The pentane is conducted through pipe 98 to condenser I9 and thence via line |08 into accumulator 10|. The cooled solvent is then pumped to storage tank 33 by way of valve |82, pump |93, line |94, and line E2. The pentanefree mercaptan product stream is withdrawn from column 98 through valve |05 and line |06 to the desired utilization.

The process of the present invention is especially directed toward the recovery of mercaptans from relatively concentrated solutions in hydrocarbons as distinguished from the low mercaptan concentrations of sour petroleum dlstillates. More particularly, concentrates produced in the synthesis of mercaptans from hydrogen sulfide and olens are suitable for extractive treatment. However, similar mercaptan concentrates from different sources are also amenable to treatment by my process. Although the alkyl mercaptans are ordinarily of greatest interest, the alkenyl, cycloaliphatic, and various substituted mercaptans are likewise recoverable. Ordinarily, hydrocarbon-mercaptan blends having mercaptan contents ranging up to 75 per cent by weight or even higher can be profitably treated for mercaptan recovery, while the most economically favorable concentrations are those of about 20 to 60 per cent based on RSH assay or from about 4 to about l2 per cent based on mercaptan sulfur content. In the case of a very high mercaptan content, say to 90 per cent or more, the concentrate may advantageously be diluted down to a lower mercaptan content with pentane or'other suitable hydrocarbon in order to facilitate handling in the primary extraction. Or, less preferably, the primary extraction may serve to dissolve the mercaptan concentrate substantially completely, with the multistage secondary extraction serving to remove neutral oil and give a pure oil-free mercaptan product.

'I'he solvent employed in the primary extraction of the present invention is an aqueous alcoholic alkali metal hydroxide solution, strong with respect to the hydroxide, and having a water-alcohol ratio selected within critical limits. I prefer to use sodium hydroxide-methanol-water solutions. While similar solutions have been used before in the extraction ot mercaptan impurities from gasoline, a single composition of solution sumced for such use. In contrast, I have found that no single solution is adequate for the total range of sulfur concentrations usually encountered in the processing of mercaptan concentrates by this invention. With a given quantity of alkali hydroxide per unit of sulfur content of the mercaptan concentrate (a matter which will be discussed later in detail), the volume ratio of water to methanol in the solvent has been found to be extremely critical in the successful operation of the primary extraction step. Thus, if this ratio is too low an excessive quantity of neutral oil will be absorbed along with the mercaptan, while if this volume ratio is too high inadequate mercaptan absorption will occur. I have found that the optimum solvent is one which will absorb from at least about 65 on up to 'l0 weight per cent of the mercaptan sulfur from the crude mercaptan oil in a single-stage equilibrium extraction at a selected temperature level, preferably within the range of about to about 120 F., when a sodium hydroxide :total sulfur weight ratio of 2:1 is used. Such a solvent etlects 90 to 95 per cent sulfur absorption in a three-stage countercurrent primary extraction of the type described above with reference to the drawing. Further, the rich solvent obtained from the three-stage operation, with conditions which gave 65 per cent sulfur absorption in one stage. showed the highest recovery of pure mercaptans per pound of sodium nvdroxlde in the solvent. If a true countercurrent extraction, rather than a three-stage contacting, is used, the same solvent is best, and extremely high through-put rates have been found feasible in using a packed tower and still eil'ecting substantially complete sulfur removal.

The temperaturerange of 40 to 120 F. encompasses the readily attainable atmospheric and near-atmospheric temperatures. While one might operate outside this range, it is not practical to do so, and the advantageous efficiencies oi' my process would thereby be greatly minimized or lost.

The water to methanol volume ratios used range from 0.8:1 to 1.5:1. The sodium hydroxide content thereof preferably ranges from 36 weight per cent in the 0.8:1 solvent down to 28 weight per cent in the 1.5:1 solvent, although considerable latitude is permissible since the volume of solvent is selected so as to give the desired 2:1 weight ratio of NaOH to sulfur in the crude mercaptan oil being treated. However, the NaOH content of the solvent is preferably always at least 20 per cent, since the use of too dilute solutions require an otherwise unnecessarily large volume of solvent. The specific gures of 36 to 28 weight per cent lust given are those obtaining when the strongest 0.8:1 solution is prepared by the preferred method of dissolving 10 pounds of sodium hydroxide in 1.057 gallons of water, and then adding 1.321 gallons of methanol while maintaining the temperature of the solution below about 140 F. This is the strongest NaOH-methanol-water solution readily made and stored at ordinary atmospheric temperatures. The other solvents of different water methanol ratios are readily prepared in the same manner, using correspondingly larger amounts of water. If in a given plant a number of crude mercaptan oils of different sulfur contents are liable to be encountered, and/or if extraction conditions are apt to vary considerably from time to time, it is moet convenient to have available a standard stock solvent which can be used per se in some instances and to which water may be added to obtain the various other water methanol ratios needed. Such stock solution is most conveniently the strong solution prepared as just described, containing 36 weight per cent NaOH and having a water methanol ratio of 0.821.

The critical interrelationships of solvent composition, feed mercaptan sulfur concentration, and temperature, adverted to above, are shown in Figure 2. This figure presents a series of curves from which the appropriate solvent water I methanol volume ratio can be read directly for operating temperatures between 20 and 120 F. and for feed stocks having mercaptan sulfur concentrations between 4 and 10 weight per cent.

To illustrate the use of Figure 2, the recovery of mercaptans from a crude Cin to C14 mercaptan concentrate having a sulfur content of 5.5 weight per cent may be considered. If it is desired to carry out the primary extraction at 100 F., reference to Figure 2 reveals that curve 3 passes through the locus of 5.5% sulfur and 100 F.

Curve 3 represents a water methanol volume ratio of 1.0:1.0. The extraction is therefore to be effected with a solvent having this ratio, utilizing a suillcient volume of the solvent that two parts by weight of NaOH are used per one part by weight of sulfur. 'I'he result is the desired 90% absorption of the mercaptans in a three-stage equilibrium contacting (curve 3 representing the solvent which gives absorption in a single stage at the given conditions).

supposing, by way of further illustration, that it is desired to do the extraction at F., rather than F., an examination of Figure 2 shows that the locus of 5.5% sulfur and 90 F. falls approximately half-way between curves 3 and I. Since curve 4 is the 1.1 :1.0 ratio, it is obvious that a solvent having a 1.05:1.0 ratio of water to meth anol will accomplish the same results as were accomplished at 100 F. with the 1.0:1.0 ratio. However, since the permissible single-stage extraction efiiciency ranges from a minimum of 65% up to a maximum of '70%. or even '75%, some latitude in the adjustment ci' water:methanol ratios is obviously allowable while still remaining within the given range. The curves in Figure 2 are so selected to take care of this latitude, and whenever the given conditions indicate a ratio between the curves, the solvent defined by the curve representing the lower water-methanol ratio may be used. Thus, while the upper limit of 'l0-'75% single-stage extraction is not set out in Figure 2, such limit is approximated by the next higher curve in the series. In other words, curve 3 represents the solvent to be used for all conditions falling on it or lying above it up to curve l, curve 4 is good for all conditions up to curve 5, et cetera. In the specific case in question, involving a 5.5% sulfur oil and a temperature oi' 90 F., the data from which Figure 2 was derived show that use of the 1.021,0 solvent (curve 3) effects 68% extraction in a single stage (which is above the lower limit oi' 65%, but below the 'l0-75% upper limit beyond which too much neutral oil is absorbed), or 94-95% extraction in the three stages.` Each curve in Figure 2 shows the maximum temperature at which that particular solvent may be used for an oil of given sulfur content. However, temperatures lower by as much as l0 to 15 F., on the average, may be used with that solvent and sulfur concentration, without dissolving so much 9 mercaptan and oil as to exceed the limits mentioned above.

In order to realize the most eilicient and economical extraction in the primary step. the solvent selected, as previously described, should be used in such volumes during the contacting periods as to contain about 2 parts by weight of sodium hydroxide or its equivalent for each part by weight of mercaptan sulfur in the charge stock. This is equivalent to a mol ratio of alkali metal hydroxide to mercaptan of 1.6:l. Using this ratio, the oil to solvent volume ratio may vary from about 3:1 for mercaptan oils containing 4 per cent mercaptan sulfur to about 8:1 for oils having 10 per cent mercaptan sulfur, when using the above-described stock solution or dilutions thereoi. I have found that the 2:1 weight ratlos for NaOH S is the minimum that can be used with practical efllciency, while the use oi' a higher ratio gives only very little improved results. A plotting of weight ratio of sodium hydroxide to sulfur content against the percentage sulfur extracted shows a remarkably sharp break at the 2:1 ratio.

Turning now to the secondary extraction, in which the residual oil and the.m captans are separately removed from the ric caustic-alcohol-water solvent, the preferred secondary solvents to be used for this purpose are the lowboiling hydrocarbons such as propane. butlle, isobutane. pentane, isopentane and selected fractions of natural gasoline. Conversely, if the process is applied to light mercaptans, heavier hydrocarbons of boiling range dissimilar to that of the mercaptans and of the oil in the crude mercaptan charge may be employed, from which the mercaptans may be iiashed or distilled overhead. The solvents used are ordinarily parafflnic, but may be partly or totally composed of oleflnic, cycloparailinic, and/or aromatic hydrocarbons. Other organic liquids, such as chlorinated hydrocarbons, certain amines, etc. may also be used. The only requirement is that the secondary solvent be essentially insoluble or of only limited solubility in the aqueous alcoholic alkaline primary solvent, and substantially inert thereto and to the mercaptans under the conditions of use.

In the first stage of the secondary extraction, the mercaptan-rich caustic-alcohol solvent may advantageously be contacted with, say, an equal volume of the selected hydrocarbon or other organic secondary solvent in order to absorb the neutral oil preferentially. It is an important feature of the invention that this operation is carried out at relatively low temperatures of about 30 to about 65 F. I have found that this very effectively suppresses the extraction of mercaptans, while removing substantially all the dissolved neutral oil from the caustic solvent. Approximately iive volumes oi hydrocarbon per volume of neutral oil present in the rich solvent should be used at the aforesaid temperatures to obtain this complete removal in a single stage. In the subsequent secondary stage or stages, I ordinarily prefer to employ 2 or more volumes of hydrocarbon per volume of mercaptan-containing primary solvent in order to minimize the number of required stages. I have found that this latter mercaptan extraction operation is favored by elevated temperatures in the range of about 100 to about 150 F. or somewhat higher. Ordinarily from 80 to 90 per cent oi' the mercaptan content oi' the primary solvent is transierred to the hydrocarbon secondary solvent in one treatment under these conditions. The rcmarkable eilectiveness oi this two-stage secondary extraction will be apparent from the specinc example given hereinafter.

I have found that the recovered caustic-alcohol primary solvent can be repeatedly used in the cycle of operations oi' this invention without material loss of solvent eiliciency. However, minor losses of alcohol may be encountered in prolonged use and may require occasional additions of make-up alcohol. Thus, the residual oil remaining after extraction of mercaptans therefrom may contain, under extreme conditions, from 0.2 to 0.5 per cent alcohol. This, of course, may be recovered if desired, but this is not usually economically advantageous. The secondary solvent ordinarily carries a small amount of alcohol around in its circuit but so long as the alcohol is lower-boiling than the mercaptan, or higher-boiling if a higher-boiling secondary solvent is used, it is recovered with the secondary solvent.

The nal recovery of the mercaptan product is accomplished in most instances by simple flash distillation of the secondary hydrocarbon solvent or by conventional stripping or distillation in columns of moderate efilciency. With mercaptans having from 10 to 14 or more carbon atoms per molecule, stripping column temperatures should not exceed about 320 F. in order to prevent decomposition of the product, and temperatures above this value are completely unnecessary due to the choice of a low-boiling hydrocarbon as the secondary solvent.

The following specific example is offered in further illustration of my invention. However, this example is not to be construed as an undue limitation of the scope of the invention.

The crude mercaptan oil employed in this experiment was obtained by the catalytic addition of hydrogen sulfide to an olen polymer fraction obtained by the polymerization of rei'lnery butylenes and having l0 to 14 carbon atoms per molecule. Analysis of the mercaptan-olefin mixture showed a mercaptan sulfur content oi 6.5 per cent by weight.

The preferred solvent to be used in the primary extraction of this material was determined by referring to Figure 2 which indicates a methanol: water volume ratio of l.0:l.0 for operating temperatures in the range of 95 to 115 F. The solvent, prepared by adding the calculated volume of water to the standard stock solvent defined above, contained 0.413 g. of NaOH per milliliter. To start the countercurrent threestage extraction, 20 ml. of aqueous caustic-methanol solvent was added to each of three contactors. Fresh mercaptan oil ml.) was added to the first stage where contacting for a period of 30 seconds was completed, followed by separation of the phases. The oil phase was then introduced into the second stage contactor for similar treatment, and thence to the third stage contactor. Following this preliminary run, the solvent phase from the second stage was transferred to the first stage and the solvent from the third stage was moved into the second stage, while fresh solvent was added to the third stage in preparation for the second cycle involving another 80 ml. chargent' fresh mercaptan concentrate. This countercurrent movement of oil and solvent was continued until the eiiect of the starting solvent had been eliminated as indicated by constant volume and sulfur values in the eilluent lean oil from the third stage and a constant volume for the rich solvent from the first stage contacter. Representative equilibrium data then obtained are presented in Table I:

TABLE I Primary three-stage extraction of cnude mercaptan odi with water-methanol-sodium` hydroxide solvent The mercaptan-rich solvent phase recovered as eliiuent from the first stage of two identical runs effected as above, was subjected to a twostage secondary extraction with pentane. The first-stage secondary contacting was carried out at 63 F. using an equal volume of the pentane secondary solvent per volume of rich primary solvent. This initial extraction removed neutral oil in admixture with mercaptans, which can be retreated in the primary extraction along with the crude charge, or less preferably. in a separate primary extraction. The second stage secondary extraction involved treatment of one volume of the Water-caustic-methanol solution from the first stage with twice its volume oi' pentane at a temperature of 100 F. The equilibrium product balance from the two-stage secondary extraction after removal of pentane by distillation is given in Table II.

Tenu: II

Secondary two-stage extraction of mercaptanrich. water-methanoZ-sodium hydroxide primary solvent with pentane secondary solvent While I have described my invention in detail with the inclusion of certain specific embodiments, no undue limitations are thereby intended other than those imposed by the claims.

I claim:

1. A process for the manufacture of highboiling mercaptans which comprises eilecting the catalytic addition of hydrogen sulfide to an olefin polymer fraction having not less than and not more than 14 carbon atoms per molecule under conditions producing a crude mercaptain oil containing at least 4 weight per cent mercaptan sulfur and comprising mercaptans having not less than 10 and not more than 14 carbon atoms per molecule and the corresponding unreacted olefin, subjecting said crude mercaptain oil to extraction with a sodium hydroxide-methanol-water solvent containing at least 20 weight per cent sodium hydroxide, at a temperature within the range of 40 F. to 120 F., in quantities such that the ratio of the weight of sodium hydroxide in the solvent charged to said extraction to the weight of sulfur in the crude mercaptain oil charged to said extraction is 2:1, said solvent having a waterzmethanol volume ratio between 0.8:1.0 and 1.5:1.0 and selected so that a singlestage equilibrium contacting of said solvent and said oil at the aforesaid conditions would effect the solution in said solvent of at least 65 per cent but not more than 'l0 per cent of the mercaptans from said oil, eecting said extraction in three stages with the oil and the solvent owing countercurrently through said Stages, thereby extracting at least per cent of the mercaptans from said oil, discharging residual olenic oil from said extraction, contacting the mercaptan-rich solvent from said extraction with about ilve volumes of liquid pentane per volume of residual olefin dissolved in said rich solvent in a single stage at a temperature not greater than 65 F. thereby desorbing into said pentane substantially all of the olefin from said rich solvent, contacting the thustreated mercaptan-containing solvent with about twice its volume of pentane in a single stage at a temperature of at least F. to desorb mercaptans from said rich solvent into said pentane, and evaporating said pentane from the resulting mercaptan-pentane solution to recover said mercaptans substantiallyr free from residual olefins.

2. A process according to claim 1 further char acterized .by evaporating pentane from the pentane-olefin-mercaptan solution formed by said first contacting of pentane with mercaptan-rich solvent. thereby leaving a residual mixture of olefin and mercaptan, returning said residual mixture to further extraction with sodium hydroxide-methanol-water solvent for ultimate recovery of the mercaptan content thereof and discharge of the olefin content thereof, evaporating pentane dissolved in the mercaptan-stripped solvent and returning resulting lean solvent to contact with further amounts of crude mercaptan oil, and combining the three portions of evaporated pentane for re-use in contacting mercaptancontaining solvent as described.

3. A process for recovering mercaptans from a crude mercaptan oil containing s, hydrocarbon oil and having a mercaptan sulfur content of at least 4 weight per cent. which comprises intimately mixing said crude mercaptan oil at a temperature lying between about 40 and about F. with suilicient of a primary solvent consisting essentially of sodium hydroxide dissolved in a mixture of water and methanol to give a sodium hydroxidezsulfur weight ratio of approximately 2:1, the waterzmethanol volume ratio of said solvent lying between about 0.8: 1.0 and about 1.5:1.0 and selected so that from about 65 to about '75 per cent extraction of the mercaptan content of the crude mercaptan oil would be effected in a single-stage equilibrium contacting at said temperature, discharging undissolved mercaptandenuded hydrocarbon oil from said mixing step. withdrawing from said mixing step primary solvent containing mercaptans and some hydrocarbon oil dissolved therein and intimately contacting same with a first portion of a secondary solvent consisting essentially of a low-boiling paraflln hydrocarbon material at a temperature lying between about 30 and about 65 F. to selectively dissolve said hydrocarbon oil from said primary solvent, contacting resulting oil-denuded primary solvent with a second portion oi said secondary solvent at a temperature lying between about 100 and about F. to dissolve said mercaptans in said second portion oi secondary solvent, and recovering sald mercaptans from said secondary solvent.

4. A process according to claim 3 wherein said crude mercaptan oil comprises alkyl mercaptans 13 of at least four carbon atoms per molecule and olens.

5. A method for recovering mercaptan from solution in an aqueous alcoholic alkali metal hydroxide solvent also containing dissolved therein a neutral organic liquid, which comprises contacting said solvent with liquid hydrocarbon at a temperature not greater than 65 F. to selectively transfer said neutral organic liquid from said solvent to solution in said hydrocarbon, separating the thus-treated solvent from said hydrocarbon and contacting same with liquid hydrocarbon at a temperature of at least 100 F. to transfer said mercaptan from said solvent to solution in said hydrocarbon, and recovering said mercaptan from said hydrocarbon.

6. A method according to claim 5 wherein said mercaptan comprises an alkyl mercaptan of at least eight carbon atoms per molecule, said neutral organic liquid comprises an olefin. the alcoholic constituent of said solvent is methanol, and said hydrocarbon to which said neutral organic liquid is transferred and said hydrocarbon to which said mercaptan is transferred is in each case a low-boiling parafn.

7. A method according to claim 6 wherein said alkali metal hydroxide is sodium hydroxide.

8. A method according to claim 6 wherein said alkali metal hydroxide is potassium hydroxide.

9. A process for separating mercaptans from admixture with substantially neutral organic liquids which comprises contacting such an admixture with an aqueous alcoholic alkali metal hydroxide primary solvent to dissolve at least part of said mercaptans together with at least part of said neutral organic liquid in said primary solvent, contacting the resulting mercaptan-rich primary solvent at a low temperature not greater than about 65 F. with suilcient of a rst liquid hydrocarbon secondary solvent to selectively dissolve substantially all of said neutral organic liquid from said primary solvent, then contacting the sotreated primary solvent with a second liquid hydrocarbon secondary solvent at an elevated temperature of at least about 100 F. to transfer said mercaptans from said primary solvent to said secondary solvent, and recovering said mercaptans from said second liquid organic secondary solvent.

HARRY E. BRENNAN.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,968,842 Malisofl' Aug. 7, 1934 2,309,652 Leum Feb. 2, 1943 2,309,654 Leum Feb. 2, 1943 30 2,357,252 Berger Aug. 29, 1944 Certicate of Correction Patent No. 2,452,040.

October 26, 1948.

HARRY E. DRENNAN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 1, line 24, for the words in Whic read by which; column 2, line 18, for distillates in read dst'illates is; line 45, same column, for equeous read aqueous; column 5, line 44, for valve 11 read valve 111 and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the casein the Patent Oilice.

Signed and sealed this 3rd day of May, A. D. 1949.

THOMAS F. MURPHY,

Assistant ommiuianer of Patente.

13 of at least four carbon atoms per molecule and olens.

5. A method for recovering mercaptan from solution in an aqueous alcoholic alkali metal hydroxide solvent also containing dissolved therein a neutral organic liquid, which comprises contacting said solvent with liquid hydrocarbon at a temperature not greater than 65 F. to selectively transfer said neutral organic liquid from said solvent to solution in said hydrocarbon, separating the thus-treated solvent from said hydrocarbon and contacting same with liquid hydrocarbon at a temperature of at least 100 F. to transfer said mercaptan from said solvent to solution in said hydrocarbon, and recovering said mercaptan from said hydrocarbon.

6. A method according to claim 5 wherein said mercaptan comprises an alkyl mercaptan of at least eight carbon atoms per molecule, said neutral organic liquid comprises an olefin. the alcoholic constituent of said solvent is methanol, and said hydrocarbon to which said neutral organic liquid is transferred and said hydrocarbon to which said mercaptan is transferred is in each case a low-boiling parafn.

7. A method according to claim 6 wherein said alkali metal hydroxide is sodium hydroxide.

8. A method according to claim 6 wherein said alkali metal hydroxide is potassium hydroxide.

9. A process for separating mercaptans from admixture with substantially neutral organic liquids which comprises contacting such an admixture with an aqueous alcoholic alkali metal hydroxide primary solvent to dissolve at least part of said mercaptans together with at least part of said neutral organic liquid in said primary solvent, contacting the resulting mercaptan-rich primary solvent at a low temperature not greater than about 65 F. with suilcient of a rst liquid hydrocarbon secondary solvent to selectively dissolve substantially all of said neutral organic liquid from said primary solvent, then contacting the sotreated primary solvent with a second liquid hydrocarbon secondary solvent at an elevated temperature of at least about 100 F. to transfer said mercaptans from said primary solvent to said secondary solvent, and recovering said mercaptans from said second liquid organic secondary solvent.

HARRY E. BRENNAN.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Number Name Date 1,968,842 Malisofl' Aug. 7, 1934 2,309,652 Leum Feb. 2, 1943 2,309,654 Leum Feb. 2, 1943 30 2,357,252 Berger Aug. 29, 1944 Certicate of Correction Patent No. 2,452,040.

October 26, 1948.

HARRY E. DRENNAN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 1, line 24, for the words in Whic read by which; column 2, line 18, for distillates in read dst'illates is; line 45, same column, for equeous read aqueous; column 5, line 44, for valve 11 read valve 111 and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the casein the Patent Oilice.

Signed and sealed this 3rd day of May, A. D. 1949.

THOMAS F. MURPHY,

Assistant ommiuianer of Patente. 

